Use streams for histogram build.

[trivialfis]

Minor optimization for building deep trees.

todo: test extmem.

[Copilot]

Pull Request Overview

This PR optimizes histogram building for deep trees by using GPU streams instead of directly passing CUDA contexts. The change allows for better concurrency and resource utilization during histogram construction.

Key changes:

• Modified histogram building methods to accept stream references instead of CUDA contexts
• Added stream pool for managing multiple concurrent histogram builds
• Updated all test cases to use the new stream-based API

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
tests/cpp/tree/gpu_hist/test_histogram.cu	Updated test calls to use stream references for histogram building
src/tree/updater_gpu_hist.cu	Added stream pool and event synchronization for concurrent histogram builds
src/tree/gpu_hist/histogram.cuh	Changed BuildHistogram method signature to accept stream reference
src/tree/gpu_hist/histogram.cu	Updated histogram building implementation to use streams

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